Assembly of atoms in a stable form. Atoms cannot form molecules under the extreme conditions found in hot stars, but they are found in cooler stars, and elsewhere throughout the universe (interstellar molecules). Molecules are hard to identify by means of the same spectroscopic techniques used to uncover atoms in stars, and infrared astronomy comes into its own for molecular studies because of the way in which many of the atom to atom bonds which characterise molecules can be examined in the infrared.

The smallest particle of a compound that retains all of its properties. Molecules are composed of two or more atoms, which may or may not be of the same element. For instance, molecular oxygen is O2, while water is H2O. Molecules are overall neutral in charge, and if charged would be a type of ion.